Shell And Tube Heat Exchangers

ABSTRACT

A process for exchanging heat in a shell and tube gas-to-gas heat exchanger between a plurality of gases, said process comprising passing a cold first gas in parallel flow to a second hot gas to provide a warmer first gas; and passing said warmer first gas in counter-current flow to a hot third gas to provide a cooler said third gas. The invention provides increased minimum tube wall temperature within the exchanger for given process conditions while maintaining a high log mean temperature differential allowing for the prevention of corrosion from entrained corrosive vapours or entrained corrosive mist with a minimal increase in effective area.

FIELD OF THE INVENTION

This invention relates to shell and tube heat exchangers, morespecifically to exchangers operating in service where a standard counterflow shell and tube heat exchanger would not be able to meet therequired process conditions without experiencing dew point corrosion,more specifically to exchangers using a combination of counter flow andparallel flow throughout the exchanger to reduce the potential for dewpoint corrosion while being able to maintain a high thermal efficiencywith only a minimal increase in effective area, and most specifically togas to gas heat exchangers used to cool hot gases containing sulphurtrioxide and/or acid vapour or heating cold gases containing sulphurdioxide and/or entrained acid mist.

BACKGROUND OF THE INVENTION

The invention relates to heat exchangers operating in potentiallycorrosive or high fouling conditions where said corrosion or foulingrates are highly dependent upon the tube wall temperature throughout theexchanger. Shell and tube exchangers are generally the preferred layoutof exchangers when fouling is expected.

Dew point corrosion is a well known phenomenon in heat exchangersdealing with a condensable corrosive vapour. When the shell or tube walltemperature on the corrosive side of the heat exchanger falls below saidcorrosive vapour's dew point, there is a potential for corrosion. Thiscan lead to fouling which causes a decrease in performance, an increasein pressure drop, and premature failure of the heat exchanger.

In sulphuric acid manufacturing, both gas streams in a heat exchangerare often potentially corrosive. The hot gas stream typically containssulphur trioxide (SO₃) and acid vapour which will rapidly corrode carbonsteel if the walls drop below the acid vapour dew point temperature. Thecold gas stream may be composed of various gas streams, including butnot limited to ambient air, dried air, dried air containing sulphurdioxide (SO₂) gas, or SO₂ gas. The cold gas stream may also containentrained acid mist from the upstream process. This entrained acid mistcan rapidly corrode the heat exchanger when it comes into direct contactwith the tube walls, especially when said tube walls are directlyimpacted with droplets of entrained acid mist within the cold inlet gas.

In hydrocarbon power plants, it is beneficial to recover as much heat aspossible from tail gas before releasing it to atmosphere. This is donethrough the use of preheat exchangers which transfers residual heat fromcombustion gas to preheat the combustion air or other fluids. The wastegas contains, amongst other gases, moisture and small quantities of SO3which combine to form sulphuric acid. Therefore, these preheatexchangers often experience dew point corrosion issues similar to thosein the production of sulphuric acid.

Counter-flow exchangers are widely preferred due to their high Log MeanTemperature Differential (LMTD). These exchangers transfer heat from ahot fluid to a cold fluid, with the hot fluid flowing longitudinallyalong the effective length of the exchanger a cold fluid flowing in theopposite direction along the longitudinal length of the exchanger, thetwo fluids separated by a barrier or barriers, generally with saidbarrier being a tube wall or tube walls. Counter flow exchangers havethe highest LMTD of the standard heat exchanger arrangements andtherefore require less effective heat transfer area to attain anequivalent heat duty to other heat exchanger designs with equivalentprocess requirements.

Counter flow exchangers have traditionally been designed to prevent dewpoint corrosion by limiting the exchanger's heat duty. Limiting the heatduty can prevent the minimum tube wall temperature from falling beneaththe dew point but may prevent the exchanger from being able to meet itsprocess requirements. An alternative to limiting the effectiveness ofthe exchanger is to increase the gas inlet temperature, although forprocess reasons this may not be desirable or feasible.

Various prior art exchangers have attempted to overcome the difficultieswith corrosion associated with the standard counter flow design whilemaintaining a high LMTD. Many of these will be familiar to a personskilled in the art and are discussed in standard sources of heatexchanger literature.

Corrosion resistant materials of construction are commonly used whenthere is a potential for dew point corrosion. Use of these materialsreduces the effects of corrosion but does not prevent the formation ofdew within the exchanger. The capital cost of the exchanger can increasesignificantly by using corrosion resistant materials depending on thematerials required, and yet the material may still experience aconsiderable amount of corrosion and fouling.

Some counter flow exchangers contain a separate sacrificial tube bank onthe cold side. These exchangers are designed so that any expected acidmist and dew point corrosion is contained to the sacrificial region. Theuse of a sacrificial tube bank does not inhibit corrosion and insteadattempts to limit its long term effects by having a separate replaceablesection.

Parallel flow exchangers are able to maintain more consistent tube walltemperatures than counter flow exchangers for equivalent inlet andoutlet conditions. These exchangers transfer heat from a hot fluid to acold fluid, with the hot fluid flowing longitudinally along theeffective length of the equipment and exchanging heat indirectly with acold fluid flowing in a generally parallel direction to the hot fluid,the two fluids separated by a barrier or barriers, generally with saidbarrier being a tube wall or tube walls. The temperatures of the twostreams approach asymptotically and converge towards a commontemperature which in turn limits the maximum heat duty and exittemperatures. A parallel flow exchanger will always have a lower LMTDthan a comparable counter flow exchanger. There is also a potential forsignificant thermal differential stresses in the inlet region where thetemperature difference between the hot and cold gases is greatest.Because of these traits, pure parallel flow exchangers are notpreferable when there is a requirement for high heat duties or wherethere is a large inlet temperature differential.

The herein disclosed invention offers a novel improvement over the priorart through a unique combination of counter flow and parallel flowsections with other additional features, whose design and use willbecome apparent after a full review of this disclosure.

SUMMARY OF THE INVENTION

The present invention provides a shell and tube heat exchangersutilizing an improved flow combination of parallel flow and counter flowto retain a high LMTD and increase the minimum tube wall temperature incomparison to a counter flow heat exchanger operating under identicalprocess conditions. This exchanger is particularly well suited for theprevention of dew point corrosion and damage from entrained acid mist.

The exchanger generally comprises two main sections with one sectionhaving a generally parallel flow arrangement and the other having agenerally counter flow arrangement between the two fluids. Partiallycooled hot gas transfers heat in a generally parallel flow manner to acold gas through the tube walls in the colder of the two sections, whilehot gas transfers heat in a generally counter flow manner to a partiallyheated cold gas through the tube walls in the hotter of the twosections. These two sections may be separated by a transition sectionwhere the flows alternate between shell side and tube side. Thistransition is particularly beneficial in plants that have unsteadyprocess conditions. Dividing the exchanger into two sections allows forfull control over the thermal design of the system, the inclusion of apartial gas by-pass or intermediate gas addition, and easier repairs ifnecessary. By alternating the shell side and tube side gas streams theoverall difference in thermal growth between the shell and tubes isreduced, which in turn reduces the stresses caused by differentialthermal growth and reduces fatigue stresses from thermal cycling.

Accordingly, in one aspect the invention provides a process forexchanging heat in a shell and tube gas-to-gas heat exchanger between aplurality of gases,

said process comprising

passing a cold first gas in parallel flow to a second hot gas to providea warmer first gas; and

passing said warmer first gas in counter-current flow to a hot third gasto provide a cooler said third gas.

Preferably, said second hot gas comprises said cooler said third gas.

In alternative embodiments, the invention as hereinabove definedprovides a process comprising passing said cold first gas as ashell-side gas, and said warmer said first gas as a tube-side gas or ashell-side gas.

In further embodiments, the invention as hereinabove defined provides aprocess comprising passing said cold first gas as a tube-side gas andsaid warmer said first gas as a tube-side gas or a shell-side gas.

In yet further embodiments, the invention as hereinabove definedprovides a process comprising removing a portion of said warmer saidfirst gas from said heat exchanger.

In still yet further embodiments, the invention as hereinabove definedprovides a process comprising removing a portion of said cooler saidthird gas from said heat exchanger.

In yet further embodiments, the invention as hereinabove definedprovides a process comprising feeding a portion of said hot third gas inadmixture with said cooler said third gas in parallel flow to said coldfirst gas.

In yet further embodiments, the invention as hereinabove definedprovides a process comprising feeding a portion of said cold first gasin admixture with said warmer said first gas in counter-current flow tosaid hot third gas.

In yet further embodiments, the invention as hereinabove definedprovides a process comprising said hot first gas comprises sulphurtrioxide.

In yet further embodiments, the invention as hereinabove definedprovides a process comprising hot first gas further comprises entrainedcorrosive liquid droplets.

In yet further embodiments, the invention as hereinabove definedprovides a process wherein said hot first gas comprises entrainedcorrosive liquid droplets.

In yet further embodiments, the invention as hereinabove definedprovides a process wherein said cold first gas comprises entrainedcorrosive liquid droplets.

In further embodiments, the invention as hereinabove defined provides aprocess wherein said second cold gas comprises air.

In a further aspect, the invention provides a process for themanufacture of sulphuric acid by the contact process comprising aprocess as hereinabove defined.

In a further aspect, the invention provides a process for exchangingheat in a shell and tube gas-to-gas heat exchanger between a first hotgas and a cold second gas as hereinabove defined in hydrocarbon powergeneration plants.

In a further aspect, the invention provides a gas-to-gas heat exchangercomprising

a shell and tube first section comprising

means for receiving a cold first gas;

means for receiving a hot second gas; and

means for passing said cold first gas in parallel flow to said hotsecond gas to provide a warmer said first gas;

a shell and tube second section comprising

means for receiving a hot third gas;

means for receiving said warmer said first gas; and

means for passing said hot third gas in counter-current flow to saidwarmer said first gas to provide a cooler said third gas.

In further embodiments, the invention provides a heat exchanger ashereinabove defined wherein said means for receiving said hot second gascomprises means for receiving said cooler said third gas.

In yet further embodiments, the invention provides a heat exchanger ashereinabove defined wherein said cooler said third gas constitutes saidhot second gas.

Having the cold inlet gas flow in parallel to partially cooled hot gasmaintains a higher LMTD throughout the exchanger and reduces thermallyinduced differential stresses when compared to a standard parallel flowdesign. Additional uses of this exchanger design including alternatingthe hot and cold gas streams to prevent against high temperaturecorrosion will become apparent to a person skilled in the art ofexchanger design and operation following a review of this disclosure.

The parallel flow section maintains a more consistent tube walltemperature than the counter flow section, which allows for further heatto be transferred between the two gas streams while maintaining the tubewall temperature above the dew point. Maintaining the tube walltemperature above the dew point prevents corrosive vapours present inthe hot gas stream from condensing. This in turn allows for the use ofstandard materials of construction as opposed to corrosion resistantmaterials, thus reducing the capital cost of the exchanger whilesimultaneously extending its expected life.

In the counter flow section the two gases flow in a generally oppositedirection to maximize heat transfer efficiency. By partially heating thecold gas in the parallel flow section, the counter flow section can bedesigned so that the tube wall temperatures remain above the dew pointtemperature of the potentially corrosive vapours. In a heat exchangerdesigned according to this invention, the coldest temperature within thecounter-flow section will always be greater than the hottest tube walltemperature within the parallel section. Therefore, dew point corrosionis inhibited in the counter flow section if it is also inhibited in theparallel flow section.

Some heat exchangers of prior art have an initial parallel flow section,with the hot inlet gas flowing first in parallel with the cold inletgas, and the remaining heat exchange occurring in a counter flowfashion. This design maintains an unnecessarily high tube walltemperature and LMTD within the parallel flow section. This limits theLMTD of the counter flow section to less than that of the parallel flowsection, therefore requiring the exchanger to have a larger effectivearea in order to reach the required heat duty than an exchanger designedaccording to the invention. Finally, the lowest tube wall temperaturewithin this prior art exchanger does not occur within the parallel flowsection and instead occurs at the cold end of the counter flow section.Therefore, even if no dew point corrosion occurs within the parallelflow section, it is still possible to have dew point corrosion withinthe counter flow section. These shortcomings are overcome by thedisclosed invention.

An exchanger designed according to the invention may be designed to havethe gases alternate sides of the tube wall during the transition betweenthe parallel flow and counter flow sections. Doing so maintains theaverage shell wall temperature of the exchanger closer to the averagetube wall temperature, thus reducing differential thermal growth.Dividing the tubes into two separate sections allows for thedifferential growth between the shell and tubes to be absorbed instages, which reduces the forces on the tube sheets. The combinedreduction in differential growth and thermally induced stresses fromalternating the shell side and tube side flows can be substantial inexchangers with a high temperature differential between their hot andcold gas streams, especially in plants with fluctuating processconditions; however, this is not required to realize the corrosionresistance benefits and relatively high heat duty capabilities of theexchanger design.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be better understood, preferredembodiments will now be described by way of example only with referenceto the accompanying drawings wherein:

FIG. 1 represents a diagrammatic cross-sectional picture of a heatexchanger according to the invention;

FIG. 2 represents a diagrammatic cross-sectional picture of an alternatearrangement of a heat exchanger according to the invention;

FIG. 3 represents a diagrammatic cross-sectional picture of an alternatearrangement of a heat exchanger according to the invention whichutilizes two separate hot gas streams;

FIG. 4 represents a diagrammatic cross-sectional picture of an alternatearrangement of a heat exchanger according to the invention whichutilizes an alternate inlet vestibule design;

FIG. 5 represents a plot of temperatures across the length of a heatexchanger according to the invention; and wherein the same numeralsdenote like parts.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a typical arrangement of a heat exchanger according to theinvention comprised of two heat exchange sections, being parallel-flowsection A and counter-flow section B.

Parallel flow section A is comprised of parallel flow shell 12,contained within which is parallel flow shell side inlet vestibule 14,parallel flow shell side 16, and parallel flow shell side outletvestibule 18 where through said parallel flow shell side 16 there passesparallel flow section tubes 20 connecting parallel flow tube side inletvestibule 22 and parallel flow tube side outlet vestibule 24.

Counter flow section B is comprised of counter flow shell 26, containedwithin which is counter flow shell side inlet vestibule 28, counter flowshell side 30, and counter flow shell side outlet vestibule 32 wherethrough said counter flow shell side 30 there passes counter flowsection tubes 34 connecting counter flow tube side inlet vestibule 36and counter flow tube side outlet vestibule 38.

Cold gas 40 enters the exchanger through parallel flow shell side inlet42 into parallel flow shell side inlet vestibule 14 passing throughparallel flow shell side 16 into parallel flow shell side outletvestibule 18 followed by parallel flow to counter flow transition duct44 into counter flow tube side inlet vestibule 36 as partially heatedcold gas 46 passing through counter flow section tubes 34 into counterflow tube side outlet vestibule 38 before exiting the exchanger asheated cold gas 48 through counter flow tube side outlet 50.

Hot gas 52 enters the exchanger through counter flow shell side inlet 54into counter flow shell side inlet vestibule 28 passing through SectionB counter flow shell side 30 into counter flow shell side outletvestibule 32 followed by counter flow to parallel flow transition upperduct 56 into parallel flow tube side upper inlet vestibule 22 aspartially cooled hot gas 58 passing through parallel flow section tubes20 into parallel flow tube side outlet vestibule 24 before exiting theexchanger as cooled hot gas 60 through parallel flow tube side outlet62. Disk baffles 64 and donut baffles 66 located throughout parallelflow shell side 16 and counter flow shell side 30 direct the shell sidefluid flow across the tubes to increase the heat transfer rate betweenthe fluids. Alternate baffle arrangements including, but not limited to,segmental baffles, double segmental baffles or an absence of baffles mayalso be used; however, disk and donut baffles combined with anaxisymmetric donut tube layout is preferred for its uniformity of heattransfer rates and thermal growth between tubes.

Separating parallel flow section A and counter flow section B whilealternating the shell-side and tube-side gas flows reduces thedifference in thermal growth between the combined growth of parallelflow shell 12 and counter flow shell 26 and the combined growth ofparallel flow tubes 20 and counter flow tubes 34. Thus, thermal cyclingloads and fatigue stresses are reduced on an exchanger according to theinvention.

Cold gas 40 may contain entrained liquid droplets as it enters theexchanger through parallel flow shell side inlet 42 which can rapidlycorrode the exchanger. Parallel flow shell side inlet vestibule 14 isdesigned such that droplets impinge on vestibule inner wall 68 wherethey accumulate harmlessly and can be drained through liquid drain 70.Parallel flow shell side inlet vestibule 14 reduces the potential forand severity of corrosion as well as the amount of fouling on theexterior of parallel flow section tubes 20 due to the previouslymentioned entrained liquid droplets when compared to allowing cold gas40 to directly enter parallel flow shell side 16 of the exchanger.

The coldest tube wall temperature in the exchanger occurs withinparallel flow section A and, thus, this section is designed to maintaina tube wall temperature above the dew point of the corrosive liquids. Aparallel flow exchanger has a higher minimum tube wall temperature thana counter flow exchanger with identical inlet and outlet conditions;therefore, parallel flow section A allows for additional heat transferwhile maintaining the tube wall temperature above the dew point whencompared to a standard counter flow exchanger. The coldest tube walltemperature within counter flow section B occurs at counter flow coldtube sheet 72 at the intersection of partially heated cold gas 46 andpartially cooled hot gas 58. The hottest tube wall temperature in theexchanger is found at counter flow hot tube sheet 74 where hot gas 52and heated cold gas 48 intersect.

The overall length of parallel section A and counter flow section B,along with the relative number of disk baffles 64 and donut baffles 66within each section can be varied to modify the relative heat duties ofeach section. This can be used during design to alter the heat duty ofthe exchanger while maintaining control over the minimum tube walltemperatures. The number and diameter of the parallel flow section tubes20 and counter flow section tubes 34 can be varied to further alter theheat duty of each section.

FIG. 2 shows an alternate arrangement of a heat exchanger according tothe invention.

Parallel flow section A is comprised of parallel flow shell 12,contained within which is parallel flow shell side inlet vestibule 14,parallel flow shell side 16 and parallel flow shell side outletvestibule 18 where through said parallel flow shell side 16 there passesparallel flow section tubes 20 connecting counter flow section tubes 34and parallel flow tube side outlet vestibule 24. Counter flow section Bis comprised of counter flow shell 26, contained within which is counterflow shell side inlet vestibule 28, counter flow shell side 30 andcounter flow shell side outlet vestibule 32 where through said counterflow shell side 30 there passes counter flow section tubes 34 connectingcounter flow tube side inlet vestibule 36 and parallel flow sectiontubes 20. Cold gas 40 enters the exchanger through parallel flow shellside inlet 42 into parallel flow shell side inlet vestibule 14 passingthrough parallel flow shell side 16 into parallel flow shell side outletvestibule 18 followed by parallel flow to counter flow transition duct44 into counter flow shell side inlet vestibule 28 as partially heatedcold gas 46 passing through counter flow section shell side 30 intocounter flow shell side outlet vestibule 32 before exiting the exchangeras heated cold gas 48 through counter flow shell side outlet 74. Hot gas52 enters the exchanger through counter flow tube side inlet 76 intocounter flow tube side inlet vestibule 36 passing through counter flowsection tubes 34 continuing into parallel flow section tubes 20 aspartially cooled hot gas 58 continuing into parallel flow tube sideoutlet vestibule 24 before exiting the exchanger as cooled hot gas 60through parallel flow tube side outlet 62. In this arrangement, parallelflow section tubes 20 are a continuation of counter flow section tubes34. Disk baffles 64 and donut baffles 66 located throughout parallelflow shell side 16 and counter flow shell side 30 direct the shell sidefluid flow across the tubes to increase the heat transfer rate betweenthe fluids.

An exchanger arrangement as shown in FIG. 2 has an identical temperatureprofile to an exchanger arrangement as shown in FIG. 1 when thethickness and heat resistance of the tubes are negligible. Thearrangement shown in FIG. 2 maintains the shell-side and tube side flowson their respective sides throughout the length of the exchanger, whichreduces the capital cost and the initial overall pressure drop of theexchanger. It is most preferred to have an identical number and diameterof tubes in parallel flow section A and counter flow section B as thetubes run the entire length of the exchanger. This limits the overallflexibility of the initial design of the exchanger in comparison to anarrangement as shown in FIG. 1. The differential thermal growth betweenthe shell and tubes of the exchanger in FIG. 2 is on a similar scale tothat of standard counter flow exchanger. It is also not possible toreplace only the parallel flow section of the exchanger arrangementshown in FIG. 2 in contrast the arrangement shown in FIG. 1. Therefore,an exchanger arrangement as shown in FIG. 2 is better suited for steadyoperating conditions, while an exchanger arrangement as shown in FIG. 1is better suited for use in unsteady operating conditions.

FIG. 3 shows an alternate arrangement of a heat exchanger according tothe invention wherein two hot gases are used in series to warm a singlecold gas. Parallel flow section A comprises of parallel flow shell 12,contained within which is parallel flow shell side inlet vestibule 14,parallel flow shell side 16, and parallel flow shell side outletvestibule 18 where through said parallel flow shell side 16 there passesparallel flow section tubes 20 connecting parallel flow tube side inletvestibule 22 and parallel flow tube side outlet vestibule 24. Counterflow section B is comprised of counter flow shell 26, contained withinwhich is counter flow shell side inlet vestibule 28, counter flow shellside 30, and counter flow shell side outlet vestibule 32 where throughsaid counter flow shell side 30 there passes counter flow section tubes34 connecting counter flow tube side inlet vestibule 36 and counter flowtube side outlet vestibule 38. Cold gas 40 enters the exchanger throughparallel flow shell side inlet 42 into parallel flow shell side inletvestibule 14 passing through parallel flow shell side 16 into parallelflow shell side outlet vestibule 18 followed by parallel flow to counterflow transition duct 44 into counter flow tube side inlet vestibule 36as partially heated cold gas 46 passing through counter flow sectiontubes 34 into counter flow tube side outlet vestibule 38 before exitingthe exchanger as double heated cold gas 78 through counter flow tubeside outlet 50. Hot gas 52 enters the exchanger through counter flowshell side inlet 54 into counter flow shell side inlet vestibule 28passing through counter flow shell side 30 into counter flow shell sideoutlet vestibule 32 before exiting the exchanger as counter flow cooledhot gas 80 through counter flow shell side outlet 74. Second hot gas 82enters the exchanger through parallel flow tube side inlet 84 intoparallel flow tube side inlet vestibule 22 passing through parallel flowsection tubes 20 into parallel flow tube side outlet vestibule 24 beforeexiting the exchanger as parallel flow cooled hot gas 86 throughparallel flow tube side outlet 62. Disk baffles 64 and donut baffles 66located throughout parallel flow shell side 16 and counter flow shellside 30 direct the shell side fluid flow across the tubes to increasethe heat transfer rate between the fluids.

The separation between parallel flow section A and counter flow sectionB allows for the intermediate removal of counter flow cooled hot gas 80and addition of second hot gas 82. In a similar manner two cold gasstreams could be used to cool a single hot gas. Modifying the gas flowrates of hot gas 52 and second hot gas 82 alters the heat duty of theexchanger in each section independently. Other benefits will be apparentto a person skilled in the art of heat exchanger design or fabrication.

FIG. 4 shows an alternate arrangement of an exchanger similar to thatshown in FIG. 2. In this arrangement, cold gas 40 enters the exchangerthrough alternate parallel flow shell side inlet 88 into alternateparallel flow shell side inlet vestibule 90 passing through parallelflow shell side 16 into parallel flow shell side outlet vestibule 18followed by alternate parallel flow to counter flow transition duct 92into counter flow shell side inlet vestibule 28 as partially heated coldgas 46 passing through counter flow section shell side 30 into counterflow shell side outlet vestibule 32 before exiting the exchanger asheated cold gas 48 through counter flow shell side outlet 74. Hot gas 52follows an identical flow path to that described in FIG. 2 and exits theexchanger as cooled hot gas 60. Alternate parallel flow shell side inletvestibule 90 provides improved mist elimination capabilities incomparison to parallel flow shell side inlet vestibule 14 as previouslyshown in FIGS. 1 through 3. Numerous similar alternate variations areapparent to a person skilled in the art of heat exchanger design orfabrication.

FIG. 5 shows a temperature profile for an exchanger designed accordingto the invention as shown in FIG. 1, FIG. 2 or FIG. 4. This temperatureprofile will be identical for an exchanger as shown in FIG. 1, FIG. 2 orFIG. 4 when the tube wall thickness and resistance are negligible. Thetemperature profile for an exchanger as shown in FIG. 3 will also beidentical provided that additionally said second hot gas 82 is composedof counter flow cooled hot gas 80. The process conditions used arearbitrary but representative of those found in a cold reheat exchangerin a sulphuric acid plant. This process has a minimum allowable tubewall temperature 94 of 300 F for the prevention of dew point corrosion.Cold gas 40 enters the exchanger at a temperature of 165 F and is heatedin parallel flow section A to partially heated cold gas 46 at atemperature of 224 F. Following parallel flow section A partially heatedcold gas 46 is heated in counter flow section B to heated cold gas 48 ata temperature of 680 F and exits the exchanger. Hot gas 52 enters theexchanger at a temperature of 860 F. It is cooled in counter flowsection B to partially cooled hot gas 58 at a temperature of 435 F.Following counter flow section B partially cooled hot gas 58 is cooledin parallel flow section A to cooled hot gas 60 at a temperature of 380F and exits the exchanger. The minimum tube wall temperature 96 withinthe exchanger is 300 F. This is equivalent to minimum allowable tubewall temperature 94 of 300 F for the prevention of dew point corrosion.The relative heat duties of parallel flow section A and counter flowsection B can be adjusted to optimize the exchanger for its desiredservice. Increasing the relative heat duty to parallel flow section Awill increase the minimum tube wall temperature, while increasing therelative heat duty to counter flow section B will increase the overallLMTD of the exchanger which in turn decreases the required effectivearea to meet the exchanger's heat duty. A prior art counter flowexchanger operating under equivalent process conditions would have aminimum tube wall temperature of approximately 272 F, which is less thanminimum allowable tube wall temperature 94. It is, therefore, expectedthat condensation would form within the prior art exchanger, causing dewpoint corrosion.

Although this disclosure has described and illustrated certain preferredembodiments of the invention, it is to be understood that the inventionis not restricted to those particular embodiments. Rather, the inventionincludes all embodiments which are functional or mechanical equivalenceof the specific embodiments and features that have been described andillustrated.

1. A process for exchanging heat in a shell and tube gas-to-gas heatexchanger between a plurality of gases, said process comprising passinga cold first gas in parallel flow to a second hot gas to provide awarmer first gas; and passing said warmer first gas in counter-currentflow to a hot third gas to provide a cooler said third gas.
 2. A processas claimed in claim 1 wherein said second hot gas comprises said coolersaid third gas.
 3. A process as claimed in claim 1 comprising passingsaid cold first gas as a shell-side gas, and said warmer said first gasas a tube-side gas or a shell-side gas.
 4. A process as claimed in claim2 comprising passing said cold first gas as a shell-side gas, and saidwarmer said first gas as a tube-side gas or a shell-side gas.
 5. Aprocess as claimed in claim 1 comprising passing said cold first gas asa tube-side gas and said warmer said first gas as a tube-side gas or ashell-side gas.
 6. A process as claimed in claim 2 comprising passingsaid cold first gas as a tube-side gas and said warmer said first gas asa tube-side gas or a shell-side gas.
 7. A process as claimed in claim 1further comprising removing a portion of said warmer said first gas fromsaid heat exchanger.
 8. A process as claimed in claim 2 furthercomprising removing a portion of said warmer said first gas from saidheat exchanger.
 9. A process as claimed in claim 1 further comprisingremoving a portion of said cooler said third gas from said heatexchanger.
 10. A process as claimed in claim 1 further comprisingfeeding a portion of said hot third gas in admixture with said coolersaid third gas in parallel flow to said cold first gas.
 11. A process asclaimed in claim 1 further comprising feeding a portion of said coldfirst gas in admixture with said warmer said first gas incounter-current flow to said hot third gas.
 12. A process as claimed inclaim 1 wherein said hot first gas comprises sulphur trioxide.
 13. Aprocess as claimed in claim 1 wherein said hot first gas comprisesentrained corrosive liquid droplets.
 14. A process as claimed in claim 1wherein said cold first gas comprises entrained corrosive liquiddroplets.
 15. A process as claimed in claim 1 wherein said second coldgas comprises air.
 16. A process for the manufacture of sulphuric acidby the contact process comprising a process as claimed in claim
 1. 17. Aprocess for exchanging heat in a shell and tube gas-to-gas heatexchanger between a first hot gas and a cold second gas as claimed inclaim 1 in hydrocarbon power generation plants.
 18. A gas-to-gas heatexchanger comprising a shell and tube first section comprising means forreceiving a cold first gas; means for receiving a hot second gas; andmeans for passing said cold first gas in parallel flow to said hotsecond gas to provide a warmer said first gas; a shell and tube secondsection comprising means for receiving a hot third gas; means forreceiving said warmer said first gas; and means for passing said hotthird gas in counter-current flow to said warmer said first gas toprovide a cooler said third gas.
 19. A heat exchanger as claimed inclaim 18 wherein said means for receiving said hot second gas comprisesmeans for receiving said cooler said third gas.
 20. A heat exchanger asclaimed in claim 19 wherein said cooler said third gas constitutes saidhot second gas.